1. Field of the Invention
The present invention relates generally to automatic transmissions and, more particularly, to a method of diagnosing a Park/Neutral switch for an automatic transmission.
2. Description of the Related Art
Automotive vehicles are commonly provided with an automatic transmission. The automatic transmission typically includes a Park/Neutral (P/N) switch to indicate whether the automatic transmission is in a Park or Neutral state. Generally, the P/N switch is used to communicate the state of the automatic transmission to a controller for an Automatic Idle Speed (AIS) system of an internal combustion engine in the automotive vehicle. The controller needs this information in order to determine a target idle speed for the engine. If the engine is not loaded, which is the case when the automatic transmission is in the Park or Neutral State, the controller uses a low target idle speed. This low target idle speed is maintained as long as the engine operating conditions are constant. When the automatic transmission is taken out of either the Park or Neutral state, the load on the engine is enough to disturb its idle condition. In that case, the target idle speed is increased and the controller uses a high target idle speed.
One disadvantage of the above automatic transmission is that when the P/N switch fails, communication between the automatic transmission and controller is cut off. As a result, the controller will not be able to meet the engine load requirements for the idle speed. If the low target idle speed is used, the engine may stall when the automatic transmission engages. On the other hand, if the high target idle speed is used, the engine RPM may be unnecessarily high. Since the controller has no other means of detecting an increased engine load due to a change of state in the automatic transmission, dynamic selection of an appropriate idle speed will be hindered and the exhaust emissions may be higher than when the AIS system is dynamically controlled.